The present invention generally relates to a lead for implantation in a human body and for use with an implantable cardiac device, such as a pacemaker or cardiovertor/defibrillator. The present invention is more particularly directed to such a lead having a composite insulating structure formed from two different insulating materials, one overlying the other, which results in a lead having superior overall characteristics including, for example, improved mechanical properties, improved resistance to biodegradation, and improved blood surface compatibility.
Implantable leads for implantable cardiac devices, such as pacemakers and cardiovertors/defibrillators, are well known. Such a lead generally includes one or more electrodes for making electrical contact with a patient's heart, a connector for connecting the lead to the implantable device, and one or more conductors for coupling the electrode or electrodes to the connector. The lead also includes an outer insulation for electrically insulating the conductor or conductors so that only the electrodes may make electrical contact with the patient's body tissue. Ideally, the outer insulation for this intended use should possess a number preferred characteristics.
One such characteristic relates to flexibility. Generally, the more flexible a lead is, the less trauma is induced to the patient's tissues by implanted lead pressure. The flexibility of lead insulation is an important factor in the overall flexibility of a lead. Hence, it is highly desirable for the lead insulation to be flexible.
Another preferred characteristic of such insulators relates to their mechanical properties. It is preferable that such insulators have good tensile properties so that, in spite of unavoidable manipulation of a lead during implant, the structural integrity of the lead is maintained. It is further preferable that the outer insulation material have resistance to abrasive wear in the event that the lead rubs against another lead, another implanted device, or anatomical structure while in use after implantation.
Biostability is another characteristic important to implantable leads, and more particularly to the insulation material used as the lead outer insulation. Biostability relates to the ability of a lead insulation material to resist degradation in the implant (in vivo) environment. Of the many in vivo degradation mechanisms believed to exist, two such mechanisms, known to be mechanisms common to certain insulation materials, such as polyurethane, and considered to be most prominent, are environmental stress cracking (ESC) and metal ion induced oxidation (MIO). ESC is characterized by surface cracks in the insulation, believed to be produced by a combined interaction of the environment (internal body fluids) acting on the insulation, and stress on the insulation material. MIO is an accelerated degradation from reaction with metal ions, such as cobalt ions, chromium ions or the like, used alone or in alloy form in lead conductors.
Still another preferred characteristic of an insulation for implantable lead use is blood surface compatibility. This relates to the degree in which the surface provided by the insulation material contributes to the formation of blood clots around the lead. An insulation which presents a highly blood compatible surface is one which contributes little to blood clot formation. Generally, an insulator which provides a highly blood compatible surface is desirable for implantable lead applications.
Another surface phenomenon associated with implantable leads, and more particularly with the outer insulations used in such leads, is the coefficient of friction of the insulation in blood. Leads which incorporate insulators having a low coefficient of friction in blood are easier to implant because such leads more readily slide against each other and into veins and arteries. As a result, a low coefficient of friction in blood is a preferred characteristic for insulators used in forming implantable cardiac leads.
The two most common polymeric materials used for outer insulation in implantable leads today are silicone and polyurethane. Each type of material exhibits its own unique set of positive and negative properties for use in implantable cardiac leads.
Silicone exhibits superior flexibility. It also is highly biostable, being essentially impervious to ESC and MIO.
Silicone, however, does exhibit some disadvantages. For example, silicone has rather inferior mechanical properties. More particularly, it has rather poor tensile and wear characteristics. In addition, silicone does not provide a surface which is as compatible in blood as some other materials. It also has a rather high coefficient of friction in blood.
Polyurethane, for use as an insulation in implantable cardiac leads, has its own set of advantages. It has good mechanical properties in terms of tensile, toughness and wear characteristics. The mechanical properties of polyurethane are so good that leads using polyurethane as an outer insulation can be made to have thin insulator wall constructions, permitting small lead outer diameter dimensions to be obtained. It also provides a highly blood-compatible surface which can minimize clotting. It also has a very low coefficient of friction in blood, rendering polyurethane outer insulation leads much easier to slide into an artery or vein during implantation than, for example, outer insulation silicone leads.
Polyurethane, however, is not without its disadvantages for implantable lead use. It generally is not as biostable as other materials, such as silicone. Some forms of polyurethane are reportedly especially susceptible to MIO and ESC. Polyurethane, in general, and some forms of polyurethane specifically, are considered to be stiffer than desirable for implantation use.
From the foregoing, it can be seen that the prior art implantable leads, using either silicone alone or polyurethane materials alone for outer insulation, have both positive characteristics and unavoidable negative characteristics. Leads incorporating silicone insulation are comparatively biostable because silicone is resistant to ESC and MIO. 0n the other hand, they have inferior mechanical properties (tensile, toughness and wear) and provide a surface having neither high blood surface compatibility nor a low coefficient of friction in blood. Leads incorporating polyurethane insulation have good mechanical properties (tensile, toughness, wear). Further, such leads provide a highly blood-compatible and low coefficient of friction surface. However, polyurethane is not highly biostable and is generally, comparatively stiff. A lead incorporating either material alone is therefore a compromise.
As will be seen hereinafter, the present invention provides a lead for implantation in the human body which overcomes the above-noted disadvantages in the prior art. More particularly, the lead of the present invention provides a composite insulating structure, formed of at least two different materials, resulting in lead performance which capitalizes on all of the advantages of the insulating materials utilized while, at the same time, negating the disadvantages of each of the insulating materials.